Conventionally, among cooling fans mounted to engines for construction machines, a fan with a comparatively small radial diameter has been made from a synthetic resin while a fan with a large radial diameter has been made from a steel plate.
On the other hand, a hybrid fan has been developed in recent years. In the hybrid fan, a body portion of a blade is basically manufactured by resin molding, where a metallic insert is embedded at the time of molding. The insert works as an attachment member for mounting the blade to a spider. The spider is made of a high-strength metal. Since the hybrid fan is the combination of the metallic spider, the metallic insert and the plastic blade body, the hybrid fan can have advantages of both the plastic fan and the steel fan. Thus, the hybrid fan is particularly expected to be widely used as a fan whose radial diameter ranges from medium to large.
According to a structure of the hybrid fan, a stress is concentrated on a base-end-side central portion of the spider and a tip-end-side corner portion of the insert. Particularly, depending on the stress concentrated on the corner portion of the insert, the blade may be damaged. The stress concentration has been conventionally observed to be caused by a flexure applied to the blade due to a centrifugal force at the time of a fan rotation. Proposals have been made to shape the insert such that no stress concentration is caused to the corner portion even when the flexure is applied (e.g., Patent Document 1). In short, by forming the insert to be homothetic with the blade, the stress concentration is prevented.    [Patent Document 1] JP-UM-A-06-4396